1. Field of the Invention
The present invention relates to a semiconductor laser chip having a diffraction grating along a laser stripe which includes a light waveguide path, and it also relates to a method of making the semiconductor laser chips.
2. Description of the Related Art
Various kinds of semiconductor light emitting elements have been widely used these days as light sources for optical communications and disks.
A distributed feedback laser (DFB laser) having a periodic perturbation (or diffraction grating) along a light waveguide path enables oscillation to be made at a single wavelength (or under single longitudinal mode) because the wavelength of the diffraction grating can be selected. Laser created using materials of the GaInAsP/InP group has been most widely used among others as the light source for long-distance high-speed optical communications.
In the case of the DFB laser. the single longitudinal mode oscillation can be generally realized, depending upon the phase of the diffraction grating at the facet of the laser stripe.
However, the present is that the period of the diffraction grating is about 2000 and that the facets of the laser stripe are cleaved facets, and it is therefore impossible in fact to accurately control the phases of the diffraction grating at the facets of the laser stripe. Although depending upon structure parameters, the productivity of laser chips which enable the single longitudinal mode oscillation to be made is thus kept lower than 50% or usually quite low ranging from 20% to 30%.
Recently, attention is paid to a such structure that reflectivity is reduced at both cleaved facets of the laser stripe and a discontinuous section (shifted only by such a phase as corresponds to a quarter of the guide wavelength .lambda.) at the period of the diffraction grating is formed in the center of the resonator. The element of this .lambda./4 shift structure is quite advantageous for single longitudinal mode operation because the gain difference of longitudinal mode (between the lowest and second modes) is large. However, this element has a problem. Namely, the process of making the shift structure becomes extremely complicated. For example, the method of conducting the two-beam interferometric exposure while combining the positive resist with the negative one (Uko, et al: Lecture prepared for the Applied Physics Meeting, Spring, 1985: Lecture No. 29p - ZB - 15), the method of conducting the two-beam interferometric exposure while using a phase mask (Shirosaki, et al: Electronic Intelligence Communication Meeting, Semiconductor Materials Section, Autumn, 1985: Lecture No. 311: Shirosaki, et al: Electronic Intelligence Communication Meeting, Report on Studies of Light Quantum Electronics, 1985, OQE85 - 60), or the like must be employed.
In a case where normalized coupling coefficient kL is larger than 1.25, radiation mode light concentrates on the position shifted by the phase of .lambda./4. This large bias in the light density distribution of waveguide mode causes spatial hall-burning in the axial direction of the resonator (Soda, et al: Electronic Intelligence Meeting, Section on Studies of Light Quantum Electronics, OQE 87 - 7, pp.49-56, 1986). The gain difference .DELTA.a between longitudinal modes which was a large value is thus made small. In short, the capability of single longitudinal mode is greatly damaged. This causes the productivity of laser chips to be made lower. As described above, it was quite difficult to produce the DFB laser chips, which can oscillate under single longitudinal mode, with high productivity.